Oct. 16, 1884] 



NA TURE 



595 



this " Volta force" is very dependent on all non-metallic 

 contacts. 



3. In a closed chain of any substances, the resultant E.M.F. 

 is the algebraic sum of the Volta forces measured electrostatically 

 in air for every junction in the chain ; neglecting magnetic or 

 impressed E.M.F. [Verified most completely by Ayrton and 

 Perry.] 



B. — Thomson 



4. The E.M.F. in any closed circuit is equal to the energy 

 conferred on unit electricity as it flows round it. 



[Neglect magnetic or impressed E.M.F. in what follows.] 



5. At the junction of two metals any energy conferred on, or 

 withdrawn from, the current, must be in the form of heat. At 

 the junction of any substance with an electrolyte, energy may 

 lie conveyed to or from the current at the expense of chemical 

 action as well as of heat. 



6. In a circuit of uniform temperature, if metallic, the sum of 

 the E.M.F. is zero by the second law of thermodynamics ; if 

 partly electrolytic, the sum of the E. M. F. is equal to the sum 

 of the energies of chemical action going on per unit current per 

 second. 



7. In any closed conducting circuit the total intrinsic E.M F. 

 is equal to the sum of the chemical actions going on per unit 

 electricity conveyed (2. JSt), diminished by the energy expended 

 in algebraically generating reversible heat. 



8. The locality of any E.M.F. may be detected, and its 

 amount measured, by observing the reversible heat or other form 

 of energy there produced or absorbed per unit current per 

 second. [This is held by Maxwell, but possibly not by 

 Thomson, though its establishment is due to him.] 



II. — Statements believed by O. J. L. to be false 



THOUGH ORTHODOX 



9. Two metals in air or water or dilute acid, but not in 

 contact, are practically at the same potential. [Sir Win, 

 Thomson, Clifton, Pellat.] 



ro. Two metals in contact are at seriously different potentials 

 (i.e. differences of potential greater than such milli-volts as are 

 concerned in thermo-electricity). [This is held by nearly every- 

 body. ] 



11. The contact force between a metal and a dielectric, or 

 between a metal and an electrolyte, is small. [Ayrton and 

 Perry, Clifton, Pellat, and probably Sir Wm. Thomson]. 



III. — Statements believed by O. J. I., to be true 



THOUGH NOT ORTHODOX 



12. A substance immersed in any medium tending to act up m 

 it chemically will (unless it is actually attacked) be at a different 

 potential to the medium in contact with it, positive if the active 

 element in the medium is electro-positive, negative if the active 

 element is electro-negative. 



13. The above difference of potential can be calculated appro- 

 ximately from the potential energy of combination between the 

 substance and the medium, the energy being measured by com- 

 pelling the combination to occur and observing the heat 

 produced per amount of substance corresponding to one unit of 

 electricity. 



14. In addition to this contact force, due to potential 

 chemical action or chemical strain, there is another which is 

 independent of chemical properties, but which seems :o be 

 greatest for badly-conducting solids, and which is in every case 

 superposed upon the former contact force, the two being 

 observed together and called the Volta effect. Very little is 

 known about this latter force except in the case of metals ; and 

 in these it varies with temperature, and is small. In the case 

 of non-metals it is often much larger than the chemical contact 

 force. 



15. The total contact force at any junction can be experi- 

 mentally determined by measuring the reversible energy deve- 

 loped or absorbed there per unit quantity of electricity conveyed 

 across the junction. [Practical difficulties, caused by irreversible 

 disturbances, being supposed overcome.] 



16. In a chain of any substances whatever, the resultant 

 E.M.F. between any two points is equal to the sun of the true 

 contact forces acting across every section of the chain between 

 the given points (neglecting magnetic or impressed forces). 



17. In a closed chain the sum of the " Volta forces," mea- 

 sured electrostatically in any (the same) medium, is equal to the 

 sum of the true contact forces, whether each individual Volta 

 force be equal to each individual true force or not. 



iS. Wherever a current flows across a seat of E.M.F., there 

 it must gain or lose energy at a rate numerically equal to the 

 E.M.F. multiplied by the strength of the current. 



Development of the above and Special Application to Metals'. 



19. A metal is not at the potential of the air touching it, but 

 is always slightly below that potential by an amount roughly 

 proportional to its heat of combustion, and calculable, at any 

 rate approximately, from it. For instance, clean zinc is pro- 

 bably about i - 8 volts below the air, copper about '8 volts 

 below, and so on. If an ordinary oxidising medium be sub- 

 stituted for "air" in the above statement it makes but little 

 difference. 



20. Two metals put into contact reduce each other instantly to 

 practically the same potential, and consequently the most oxidis- 

 able one receives from the other a positive charge, the effect of 

 which can be observed electrostatically. 



21. There is a slight true contact force at the junction of two 

 metals which prevents their reduction to exactly the same poten- 

 tial, but the outstanding difference is small, and varies with tem- 

 perature. It can be measured thermo-electrically by the Peltier 

 effect, but in no other known way. It is probably entirely 

 independent of surrounding media, metallic or otherwise. 



22. If two metals are in contact the potential of the medium 

 surrounding them is no longer uniform : if a dielectric it is in a 

 state of strain, if an electrolyte it conveys a current. 



23. In the former case the major part of the total difference of 

 potential is related closely to the difference of the potential 

 energies of combination, and is approximately calculable there- 

 from. In the latter case the total E.M.F. is calculable accurately 

 from the energy of the chemical process going on, minus or plus 

 the energies concerned in reversible heat effects. 



24. There are two distinct and independent kinds of series in 

 which the metals (and possibly all solids) can be placed ; one 

 kind depends on the dielectric or electrolytic medium in which 

 the bodies are immersed ; the other kind depends on tempera- 

 ture. The one is something like the Volta series, but it is 

 really the Volta series minus the Peltier ; the other is the 

 Peltier. To reckon up the total E.M.F. of a circuit we may 

 take differences of numbers from each series and add them 

 together. 



IV.— Brief Summary of the Argument 



25. Wherever a current gains or loses energy, there must be a 

 seat of E.M.F. ; and conversely, wherever there is a seat of 

 E.M.F., a current must lose or gain energy (that is, must gene- 

 rate or destroy some other form of energy, chemical, thermal, or 

 other) in passing it. 



26. A current gains no energy (i.e. destroys no heat) in cross- 

 ing from copper to zinc, hence there is no appreciable E.M.F. 

 there. 



27. When a current flows from zinc to acid, the energy of the 

 combination which occurs is by no means accounted for by the 

 heat there generated, and the balance is gained by the current ; 

 hence at a zinc-acid junction there must be a considerable 

 E.M.F. (say at a maximum 2'3 volts). 



28. A piece of zinc immersed in acid is therefore at a lower 

 potential than the acid, though how much lower it is impossible 

 to say, because no actual chemical action occurs. [If chemical 

 action does occur, it is due to impurities, or at any rate to local 

 currents, and it is of the nature of a disturbance.] 



20. A piece of zinc, half in air and half in water or acid, 

 causes no great difference of potential between the air and the 

 water (Thomson, Clifton, x\yrton and Perry, &c), consequently 

 air must behave much like water. 



30. If the air were slightly positive to the water, as it is 

 (Hankel), it might mean that the potential energy of combination 

 of air with zinc is slightly greater than that of water, or it might 

 represent a difference in the thermo-electric contact forces be- 

 tween zinc and air, and zinc and water, or it might depend on a 

 contact force between air and water. [If such a contact force 

 between air and water exists, it is obviously of great importance 

 in the theory of atmospheric electricity, for the slow sinking of 

 mist globules through the air would render them electrical.] 



31. Condenser methods of investigating contact force no more 

 avoid the necessity for unknown contacts than do straightforward 

 electrometer or galvanometer methods ; the circuit is completed 

 by air in the one 1 ase and by metal in the other, and the E.M.F. 

 of an air contact i- more hopelessly unknown than that of a 

 metal contact. 



